Laser technology has advanced significantly in the recent years and now it is possible to produce fiber based semiconductor laser systems with a tolerable efficiency which can be used in cold ablation, for example.
The optical fibers in fiber lasers for transmitting the laser beam are not, however, suitable for transmitting high-power, pulse-compressed laser beams to the work spot. The fibers simply cannot withstand the transmission of the high-power pulse. One reason as to why optical fibers have been introduced in laser beam transmission is that the transmission of a laser beam from one place to another through free air space by means of mirrors to the work spot is in itself extremely difficult and fairly impossible to accomplish with precision on an industrial scale. Furthermore, impurities in the air and, on the other hand, scattering and absorption mechanisms in the component parts of the air may bring about losses in the laser power which will affect the beam power at the target in a manner difficult to predict. Naturally, laser beams propagating in free air space also pose a significant safety risk.
Competing with the fully fiber based diode pumped semiconductor laser is the lamp pumped laser source in which the laser beam is first conducted into the fiber and thence further to the work spot. According to the information available to the applicant on the priority date of the present application these fiber based laser systems are at the moment the only way to bring about laser ablation based production on an industrial scale.
The fibers of present-day fiber lasers and, hence, the limited beam power impose limitations as to which materials can be vaporized. Aluminum as such can be vaporized using a reasonable pulse power, whereas materials more difficult to vaporize, such as copper, tungsten etc., require a pulse power considerably higher.
The same applies into situation in which new compounds were in the interest to be brought up with the same conventional techniques. Examples to be mentioned are for instance manufacturing diamond directly from carbon or alumina production straight from aluminium and oxygen via the appropriate reaction in the vapour-phase in post-laser-ablation conditions.
There are other problems, too, associated with the fiber laser technology. For example, large amounts of energy cannot be transmitted through optical fiber without the fiber melting and/or breaking or without substantial degradation of the laser beam quality as the fiber becomes deformed due to the high power transmitted. Already a pulse power of 10 μJ may damage the fiber if it has even the slightest structural or qualitative weaknesses. In fiber technology, especially prone to damage are the fiber optic couplers, which, for example, connect together a plurality of power sources, such as diode pumps.
The shorter the pulse, the bigger the amount of energy in it, so therefore this problem becomes more aggravated as the laser pulse gets shorter. The problem manifests itself already in nanosecond pulse lasers.
When employing novel cold-ablation, both qualitative and production rate related problems associated with coating, thin film production as well as cutting/grooving/carving etc. has been approached by focusing on increasing laser power and reducing the spot size of the laser beam on the target. However, most of the power increase was consumed to noise. The qualitative and production rate related problems were still remaining although some laser manufacturers resolved the laser power related problem. Representative samples for both coating/thin film as well as cutting/grooving/carving etc could be produced only with low with repetition rates, narrow scanning widths and with long working time beyond industrial feasibility as such, highlighted especially for large bodies.
The pulse duration decrease further to femto or even to atto-second scale makes the problem almost irresolvable. For example, in a pico-second laser system with a pulse duration of 10-15 ps the pulse energy should be 5 μJ for a 10-30 μm spot, when the total power of the laser is 100 W and the repetition rate 20 MHz. Such a fibre to withstand such a pulse is not available at the priority date of the current application according to the knowledge of the writer at the very date.
In laser ablation, which is an important field of application for the fiber laser, it is, however, quite important to achieve a maximal and optimal pulse power and pulse energy. Considering a situation where the pulse length is 15 ps and the pulse power is 5 μJ and the total power 1000 W, the energy level of the pulse is about 400,000 W (400 kW). According to the information available to the applicant on the priority date of the application, no-one has succeeded in manufacturing a fiber which would transmit even a 200-kW pulse with a 15-ps pulse length and with the pulse shape remaining optimal.
Nevertheless, if unlimited facilities are desired for plasma production from any substance available, the power level of the pulse should be freely selectable, for instance between 200 kW and 80 MW.
The problems associated with present-day fiber lasers are not, however, solely limited to the fiber, but also to the coupling of separate diode pumps by means of optical couplers in order to achieve a desired total power, the resulting beam being conducted through one single fiber to the work spot.
The applicable optical couplers also should withstand as much power as the optical fiber which carries the high power pulse to the work spot. Furthermore, the pulse shape should remain optimal in all stages of transmission of the laser beam. Optical couplers that withstand even the current power values are extremely expensive to manufacture, they have rather a poor reliability, and they constitute a part susceptible to wear, so they require periodic replacing.
The production rate is directly proportional to the repetition rate or repetition frequency. On one hand the known mirror-film scanners (galvano-scanners or back and worth moving type of scanners), which do their duty cycle in way characterized by their back and forth movement, the stopping of the mirror at the both ends of the duty cycle is somewhat problematic as well as the accelerating and decelerating related to the turning point and the related momentary stop, which all limit the utilizability of the mirror as scanner, but especially also to the scanning width. If the production rate were tried to be scaled up, by increasing the repetition rate, the acceleration and deceleration cause either a narrow scanning range or uneven distribution of the radiation and thus the plasma at the target when radiation hit the target via accelerating and/or decelerating mirror.
If trying to increase the coating/thin film production rate by simply increasing the pulse repetition rate, the present above mentioned known scanners direct the pulses to overlapping spot of the target area already at the low pulse repetition rates in kHz-range, in an uncontrolled way.
The same problem applies to nano-second range lasers, the problem being naturally even more severe because of the long lasting pulse with high energy. Thus, even one single nano-second range pulse erodes the target material drastically.
Prior art hardware solutions based on laser beams and ablation involve problems relating to power and quality, for example and especially in association with scanners, whereby, from the point of view of ablation, the repetition frequency cannot be raised to a level that would enable a large-scale mass production of a product of good and uniform quality. Furthermore, prior art scanners are located outside the vaporizer unit (vacuum chamber) so that the laser beam has to be directed into the vacuum chamber through an optical window which will always reduce the power to some extent.
According to the information available to the applicant, the effective power in ablation, when using equipment known at the priority date of the present application, is around 10 W. Then the repetition frequency, for instance, may be limited to only a 4-MHz chopping frequency with laser. If one attempts to increase the pulse frequency further, the scanners according to the prior art will cause a significant part of the pulses of the laser beam being directed uncontrollably onto the wall structures of the laser apparatus, and also into the ablated material in the form of plasma, having the net effect that the quality of the surface to be produced will suffer as will also the production rate and, furthermore, the radiation flux hitting the target will not be uniform enough, which may affect the structure of the plasma, which thus may, upon hitting the surface to be coated, produce a surface of uneven quality.
Then, in machining, working, too, where the target is a piece and/or part thereof to be machined worked, the surface of which is to be shaped, it easily happens that both the cutting efficiency and the quality of the cut are affected. Furthermore, there is a significant risk of spatters landing on the surfaces around the point of cut as well as on the very surface to be coated. In addition, with prior art technology, it takes time to achieve several layers with repeated surface treatment, and the quality of the end result is not necessarily uniform enough. For example, the applicant is not aware of any technology published by the priority date of the application which could be used to produce strong three-dimensional objects on a printer.
With known scanners of which the applicant is aware at the priority date of the present application the scanning speeds remain at about 3 m/s, and even then, the scanning speed is not really constant but varies during the scanning. This is because scanners according to the prior art are based on fixed turning mirrors which stop when the scanning distance has been traveled, and then move in the opposite direction, repeating the scanning procedure. Mirrors are also known which move back and forth, but these have the same problem with the non-uniformity of the movement. An ablation technique implemented with planar mirrors is disclosed in patent publications U.S. Pat. No. 6,372,103 and U.S. Pat. No. 6,063,455. Since the scanning speed is not constant, due to the acceleration, deceleration and stopping of the scanning speed, also the yield of plasma generated through vaporization at the work spot is different at different points of the target, especially at the extremities of the scanning area, because the yield and also the quality of the plasma completely depend on the scanning speed. In a sense, one could consider it as a main rule that the higher the energy level and the number of pulses per time unit, the bigger this drawback when using prior art devices. In successful ablation, matter is vaporized into atomic particles. But when there is some disturbance, target material will be released/become detached in fragments which may be several micrometers in size, which naturally affects the quality of the surface to be produced by ablation.
Since the present-day scanner speeds are low, increasing the pulse frequency would result in energy levels so high being directed onto the mirror structures that present-day mirror structures would melt/burn if the laser beam were not expanded prior to its arrival at the scanner. Therefore, a separate collecting lens arrangement is additionally needed between the scanner and the ablation target.
The operating principle of present-day scanners dictates that they have to be light. This also means that they have a relatively small mass to absorb the energy of the laser beam. This fact further adds to the melting/burning risk in present ablation applications.
In the prior art techniques, the target may not only ware out unevenly, but may also fragment easily and degrade the plasma quality. Thus, the surface to be coated with such a plasma can also suffers the detrimental effects of the plasma, as well as the fragments-flying-through-the-plasma originating anomalies in it. The surfaces as well as the cut lines may comprise fragments, plasma may be not evenly distributed to form such a coating etc. which are problematic in accuracy demanding application, but may be not experienced severely problematic, with coatings like ink, paint or decorative pigments, for instance, provided that the defects keep below the detection limit of the very application.
The present methods ware out the target in a single use so that same target is not available for a further use from the same surface again. The problem has been tackled by utilising only a virgin surface of the target, by moving target material and/or the beam spot accordingly.
In machining or work-related applications the left-overs or the debris comprising some fragments also can make the cut-line un even and thus inappropriate, as the case could for instance in flow-control drillings. Also the surface could be formed to have a random bumpy appearance caused by the released fragments, which may be not appropriate in certain semiconductor manufacturing, for instance.
In addition, the mirror-film scanners moving back and forth generate inertial forces that load the structure it self, but also to the bearings to which the mirror is attached and/or which cause the mirror movement. Such inertia little by little may loosen the attachment of the mirror, especially if such mirror were working nearly at the extreme range of the possible operational settings, and may lead to roaming of the settings in long time scale, which may be seen from uneven repeatability of the product quality. Because of the stoppings, as well as the direction and the related velocity changes of the movement, such a mirror-film scanner has a very limited scanning width so to be used for ablation and plasma production. The effective duty cycle is relatively short to the whole cycle, although the operation is anyway quite slow. In the point of view of increasing the productivity of a system utilising mirror-film scanners, the plasma making rate is in prerequisite slow, scanning width narrow, operation unstable for long time period scales, but yield also a very high probability to get involved with unwanted particle emission in to the plasma, and consequently to the products that are involved with the plasma via the machinery and/or coating.
One problem in prior-art solutions is the scanning width. These solutions use line scanning in mirror film scanners whereby, theoretically, one could think that it is possible to achieve a nominal scan line width of about 70 mm, but in practice the scanning width may problematically remain even around 30 mm, whereby the fringe regions of the scanning area may be left non-uniform in quality and/or different from the central regions. Scanning widths this small also contribute to the fact that the use of present-day laser equipment in surface treatment applications for large and wide objects is industrially unfeasible or technically impossible to implement.
FIG. 67 illustrates a situation in accordance with the prior art, with the own marking and reference numbers locally used for referring the prior art references in the figure, and where the laser beam is out of focus and the resulting plasma thus has rather a low quality. The plasma which is released may also contain fragments 116 of the target. At the same time, the target material to be vaporized may be damaged to such an extent that it cannot be used anymore. This situation is typical in the prior art when using a material preform 114, a target, which is too thick. In order to keep the focus optimal, the material preform 114 should move 117, z movement, in the direction of incidence of the laser beam 111 for a distance equivalent to the extent to which the material preform 114 is consumed. Unsolved is, however, the problem that even if the material preform 114 could be brought into focus, the surface structure and composition of the material preform 114 already will have changed, the extent of the change being proportional to the amount of material vaporized off the target 114. The surface structure of a thick target according to the prior art will also change as it wears. For instance, if the target is a compound or an alloy, it is easy to see the problem.
In arrangements according to the prior art, a change in the focus of the laser beam in the middle of ablation, relative to the material to be vaporized, will immediately affect the quality of the plasma, because the energy density of the pulse on the surface of the material will (normally) decrease, whereby vaporization/generation of plasma is no longer complete. This results in low-energy plasma and unnecessarily large amounts of fragments/particles as well as a change in the surface morphology, and possible changes in the adhesion of the coating and/or coating thickness.
Attempts have been made to alleviate the problem by adjusting the focus. When in equipment according to the prior art the repetition frequency of the laser pulses is low, say below 200 kHz, and the scanning speed only 3 m/s or less, the speed of change of the intensity of plasma is low, whereby the equipment has time to react to the change of the intensity of plasma by adjusting the focus. A so-called realtime plasma intensity measurement system can be used when a) the quality of the surface and its uniformity are of no importance or b) when the scanning speed is low.
Then, according to the information available to the applicant at the priority date of the present application, it is not possible to produce high-quality plasma using prior-art technology. Thus quite many coatings cannot be manufactured as high-quality products in accordance with the prior art.
Systems according to the prior art include complex adjustment systems which must be used in them. In current known methods the material preform is usually in the form of a thick bar or sheet. A zoom focusing lens must be used or the material preform must be moved toward the laser beam as the material preform gets consumed. Even an attempt to implement this is already extremely difficult and expensive, if at all possible in a manner sufficiently reliable, and even then the quality varies greatly, whereby precise control is almost impossible, the manufacture of a thick preform is expensive and so on.
As publication U.S. Pat. No. 6,372,103 B1 teaches, current technology can direct the laser pulse to the ablation target only as either predominately S polarized or, alternatively, predominately P polarized or circularly polarized light, and not as random polarized light.